In an assembled battery system using serially-connected battery cells, it is typical that a voltage of each individual battery cell is measured and used for control purposes. Particularly, in case of a secondary battery such as a lithium battery, a voltage of each individual battery cell needs to be correctly and accurately measured from the viewpoint of safety and performance utilization. To assure correct measurement of the battery cell voltage, it is important to reliably detect a circuit failure, especially disconnection of a voltage detecting line connected to each of the battery cells.
Methods of detecting disconnection of a voltage detecting line are selected depending on the types of circuits used. Examples of the circuits for measurement of a battery cell voltage include a method of directly measuring a battery cell voltage and another method of measuring a battery cell voltage charged in a capacitor parallel-connected to a battery cell. There are also known a number of methods in which disconnection is detected by temporarily operating the measurement circuits and determining the changing fashion of a voltage.
The method of directly measuring a battery cell voltage can determine occurrence of disconnection due to the fact that the disconnection makes it impossible to measure a voltage itself. There is proposed a method of detecting minute disconnection using the fact that when a resistor circuit is temporarily connected to a battery cell in parallel, a voltage is changed by a specified value or more if disconnection occurs. In the method of measuring a battery cell voltage charged in a capacitor, it is sometimes the case that a voltage is generated in the capacitor despite the non-occurrence of disconnection. Given this, there is proposed a method using the principle that when a resistor circuit is connected to a battery cell in parallel, a voltage measured is greatly changed if disconnection occurs.
In any of the voltage measuring methods mentioned above, if an RC-filter is connected between a voltage detecting unit and a battery cell, then it is possible to eliminate inverter noises otherwise generated at the load side of the battery. This is advantageous in increasing measurement accuracy. In the second method using a capacitor to measure a voltage, the capacitor has a noise eliminating function. However, most of the capacitors used for measurement purposes are usually small in capacity, which limits their function as a noise filter. An increased noise eliminating effect can be obtained by connecting, independently of the measuring unit, an RC filter as a combination of a large-capacity capacitor and a resistor. Thus, use of the RC filter has a merit regardless of the voltage measuring methods.
In the method using the RC filter, a resistor is connected to a voltage detecting line drawn from a battery cell and a capacitor is connected to the connection point between the resistor and the voltage detecting line in parallel with the battery cell. If disconnection occurs in the voltage detecting line, then the voltage of the corresponding battery cell should be measured as an abnormal value. In the circuit having the RC filter connected thereto, however, there is a possibility that, even when disconnection occurs in the voltage detecting line, a voltage value differing from the voltage of the battery cell may be measured due to the electric charges remaining in the capacitor of the RC filter.
The methods set forth above are applicable to a battery cell voltage measuring circuit having no RC filter and cannot be applied to a measuring circuit provided with a RC filter. In these methods, it is determined to be normal if a voltage is not changed when a short-circuiting circuit is connected. As such, these methods suffer from a problem since they cannot distinguish the non-occurrence of disconnection of the voltage detecting line from the failure of the short-circuiting circuit.